Preheat the oven to 375℉/190℃. Line a muffin tin with liners and set aside. Mash the bananas and set aside.
In a bowl, add the dry ingredients. Mix together and set aside.
1 1/2 cups all-purpose flour, 1 1/4 cups oats, 1 1/2 tsps baking powder, 1 tsp baking soda, 3/4 tsps cinnamon, 2 tsps cornstarch, 1/4 tsp salt
In a large bowl, add the sugar and eggs. Whisk together for 2-3 minutes vigorously until well combined.
3/4 cups dark brown sugar, 2 eggs
Add in the oil and bananas. Continue whisking. It may be lumpy from the bananas and that is normal.
1 cup bananas, 1/2 cup oil
Add in the sour cream and vanilla. Whisk together.
1 tbsp vanilla, 1/2 cup sour cream
Add in the dry ingredients. With a spatula, gently fold just until a soft muffin batter forms. Do not over-mix!
Into the bowl with the blueberries, add in 1 tbsp of flour. Dust the berries in the flour before adding them into the muffin batter. Then, add them in a gently mix in a few times - do not over-mix!
1 1/2 cups fresh blueberries
Scoop the batter evenly between the liners. If using regular liners, fill them all the way. If using tulip liners, fill them about halfway.
Bake the muffins for 25-35 minutes. They will be ready when a toothpick inserted in the middle comes out clean.
Allow to cool, and enjoy!